When you start a level, you should be able to pick a color yoshi. There is a reason for this. Some yoshis like different fruits, so you will get 3 hearts from eating them(that is,if you didn't get it as a lucky fruit), but every yoshi gets 3 hearts from eating a melon.

Green yoshi- watermelon
Red yoshi- apple
Yellow yoshi- bannanas
Blue yoshi- grapes
Light blue yoshi- grapes
Pink yoshi- apples